The invention generally relates to the purification of naphthalenic compounds, and more particularly relates to the purification of naphthalenedicarboxylic acids.
Polymers based on dimethyinaphthalenedicarboxylates and their corresponding acids are known to be useful in a wide variety of commercial applications. For example, films made from polymers incorporating dimethyl-2,6-naphthalenedicarboxylate (2,6-NDC) exhibit strength and thermal properties which are superior to films and fibers made from other polymers such as polyethyleneterephthalate (PET). These enhanced properties have led to the use of 2,6-NDC-based polymers in camera films and magnetic recording tapes as well as electrical and electronic components.
2,6-NDC-based polymers also exhibit high resistance to the diffusion of gases such as carbon dioxide, water vapor and oxygen. This resistance to gas diffusion makes these polymers useful in films and containers for a wide variety of food and beverage packaging applications.
The superior physical strength of 2,6-NDC-based polymers also renders these polymers useful in physically demanding applications such as cords for automobile and motorcycle tires.
The use of 2,6-naphthalenedicarboxylic acid in such applications provides several advantages over 2,6-NDC. First, the weight differences between the acid and the ester typically results in higher yields of polymer per pound of feedstock. Additionally, polymerization of the acid with ethylene glycol produces water, rather than the more difficult to handle methanol produced when the ester is polymerized with ethylene glycol. Furthermore, in polymer plants already designed to handle only acid-type monomers, use of the acid permits the addition of naphthalenedicarboxyl moieties to polymers which could not be accomplished if only the ester form was available.
The production and direct purification of an acid monomer also greatly simplifies monomer production. Specifically, in the synthesis of 2,6-NDC, 2,6-dimethylnaphthalene (2,6-DMN) is oxidized to produce 2,6-naphthalenedicarboxylic acid (2,6-NDA), which must subsequently be esterified to produce 2,6-NDC, with one or more purification steps such as distillation or recrystallization being performed in one or both of those steps as necessary to produce a product having a high yield and purity. In contrast, direct purification of the oxidation product of 2,6-DMN is relatively simple, and can therefore result in a lower cost monomer.
Unfortunately, the purification of 2,6-NDA typically has been conducted at temperatures at or in excess of 600 degrees Fahrenheit to permit the processing of a relatively high percent of total dissolved solids. Operating at these temperatures requires substantial capital investment in plants capable of withstanding the pressure resulting from operating at these temperatures. Furthermore, operating at these temperatures requires a large investment in energy to heat the reaction mixtures to the required 600+ degree Fahrenheit temperature. Finally, even when operating at these preferred conditions, impurities present in the purified monomer may require additional purification before they are useful in polymer applications.
What is needed is a relatively inexpensive way to purify naphthalenic diacid monomer more efficiently and at low cost, and which results in a lower level of impurities than is presently produced in the typical high temperature purification processes presently used.
We have found that naphthalenic diacid monomers can be efficiently produced by operating at lower temperatures and solids loadings levels than have been typically used in commercial practice. Surprisingly, when the proper catalyst and operating conditions are selected, monomer having lower levels of undesired impurities is obtained, thereby decreasing the need for further purification of the acid monomer.
Additionally, because these reactions are performed at lower temperatures, we have found that underutilized chemical plant assets designed for less strenuous operating conditions, such as those used to purify terephthalic acid, may be employed to purify diacid monomers such as 2,6-NDA. Utilizing such existing assets can eliminate capital costs associated with building a plant capable of operating at the 600+ degree Fahrenheit temperature typically used commercially for NDA purification, thereby further enabling relatively inexpensive production of the monomer.